The iMac with Retina 5K Display features two mobile GPU choices: Radeon R9 M290X and R9 M295X. Are they comparable to the desktop version of the R9 290X? No. What about the GeForce GTX 780M in the 2013 non-Retina iMac? Is it comparable to a desktop version of the GTX 780? No. Allow us to illustrate how they compare in performance . . .

Tomb Raider
Using the HIGH preset at 2560x1440, we ran the built-in benchmark. (RED graph bar means FASTEST in FRAMES per SECOND.)

Diablo III
Using the best settings with no vsync but with anti-aliasing at 2560x1440, we entered the game at the same location and observed FPS (Control R). (RED graph bar means FASTEST in FRAMES per SECOND.)

GPUtest- FurMark
This is a very intensive OpenGL benchmark that uses fur rendering algorithms to measure the performance of the graphics card. Fur rendering is especially adapted to overheat the GPU and that's why FurMark is also a perfect stability and stress test tool for the graphics card. We ran the test at 2560x1440 Windowed with anti-aliasing disabled. (HIGHER number means FASTEST in Frames Per Second.)

LuxMarkThis is an OpenCL benchmark. We rendered the Sala scene using GPUs only. (RED graph bar means FASTEST in Thousands of Samples per Second.)

Unigine Valley OpenGL Benchmark "flies" through forest-covered valley surrounded by vast mountains. It amazes with its scale from a bird's-eye view of 64 million meters of extremely detailed terrain down to every leaf and flower petal. It features advanced visual technologies: dynamic sky, volumetric clouds, sun shafts, DOF, ambient occlusion.

We used Extreme preset: 1600x900 windowed resolution with 8x Anti-aliasing, Ultra Quality for Shaders and Textures, and with Occlusion, Refraction, and Volumetric Shadows enabled. We posted AVERAGE frames per second. (HIGHER number means FASTEST in Frames Per Second.)

INSIGHTSAs you can see from the graphs, the mobile GPUs (light gray bar) are slower than the desktop GPUs (dark gray bar). That's because, though the model number may be similar, the chipsets are totally different. Therefore the core clock speed, the number of compute units, the fillrate, etc., are in no way comparable.

Why didn't Apple use desktop GPUs in the iMac? After all, it's a desktop computer. Answer: heat and power. You can't stuff a desktop GPU in a thin, all-in-one desktop and keep it running cool and silent.

Caveat: Neither the desktop Radeon R9 290X nor the desktop GeForce GTX 780 are being sold as Mac Editions. The R9 290X was an unflashed Windows PC card that happens to run under Yosemite. The GTX 780 was a flashed Windows PC GPU that was made compatible to the NVIDIA web driver released for Yosemite. And both benefit from extra boost of an aux power supply if you plan to stress them. We used these extreme GPUs to make a point. For more information on these and other 'alternate' GPUs, consult the folks at MacVidCards.

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